Immunogenetics, at the present time, is the master discipline for elucidation of differentiative events and mechanisms. Its cornerstone is the following simple formula: (1) Identify a cell component controlled by a single gene. (2) Map the gene. (3) Assemble enough examples to indicate gene sets belonging to particular chapters of the differentiative history. (4) Use immunochemical methods to ascertain the structure of the cell surface component in each case. With this information in hand, favorable subsidiary models can be selected to answer such questions as (1) What are the functional commitments of each set of immunogenetically-identified sub-populations? (2) What is the inductive signal? Where does it come from? How does it work? This research grant application exemplifies this approach for the cells of the immune system. It dwells especially on Ly genes as members of gene sets, and on Ly antigens as hallmarks of functionally distinct immunocompetent populations and of sequential compartments within particular differentiative pathways.